Iron Sulfide Melt Research Articles

Abundance of highly siderophile elements in lunar basalts controlled by iron sulfide melt

The Moon accreted meteoritic material towards the end of Solar System formation. Quantification of this late accretion requires an estimation of the abundance of highly siderophile, or iron-loving, elements in the lunar mantle. As lunar mantle samples are not available, estimates are derived from lunar basalt compositions, but the melting phase relations needed to derive the mantle composition are poorly constrained. Here we present sulfur solubility measurements from laboratory experiments, combined with thermodynamic calculations, which show that the lunar basalt source is likely to be saturated in a sulfur-poor, iron-rich sulfide melt that concentrates some highly siderophile elements more than others. We found that the observed range in the ratios of highly siderophile elements in primitive lunar basalts is much smaller than expected from residual sulfide control alone. Instead, the elemental ratios are consistent with mixing between primary sulfide-saturated melts and minute (<1%) amounts of lunar regolith that contain impact debris. Although the composition of some samples suggests a highly depleted lunar mantle, the exact level of depletion is unclear, because mixing trajectories overlap at the inferred level of regolith contamination. We conclude that the composition of the lunar mantle is veiled by regolith contamination of the lunar basalts. If so, highly siderophile element abundances in lunar mantle-derived materials cannot be used to determine the mass of material accreted late onto the Moon. The abundance of iron-loving elements in Moon rocks cannot indicate the amount of late accretion onto the Moon, according to experiments and thermodynamic calculations constraining the behaviour of these elements under lunar mantle conditions.

Effect of redox conditions on iron metal phase segregation during experimental high-temperature centrifuge modeling of the origin of the Moon’s core

The possible origin of the Moon’s metallic core at the precipitation of iron–sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %) + ferrobasalt (10 wt %) + metallic phase Fe95S5 (wt %) in a high-temperature centrifuge at 1430–1450°C. The oxygen fugacity fO2 was determined from the composition of the quenched experimental silicate melts (glasses). A decrease in fO2 is proved to be favorable for the segregation of iron–sulfide melt from the silicate matrix. The metallic phase is most effectively segregated in the form of melt droplets, and these droplets are accumulated in the lower portions of the samples under strongly reduced conditions, at fO2 ∼ 4.5–5.5 orders of magnitude lower than the iron–wustite buffer.

Volcanological and Structural Controls on Mineralization at the Mount Keith and Cliffs Komatiite-Associated Nickel Sulfide Deposits, Agnew-Wiluna Belt, Western Australia—Implications for Ore Genesis and Targeting

The Mount Keith region of the Agnew-Wiluna belt in Western Australia is perhaps best known for hosting the giant komatiite-associated disseminated Ni sulfide deposit known as MKD5. It also hosts the sizeable Cliffs massive Ni sulfide deposit, which is the eighth largest in the Archean Yilgarn block. A concerted program of regional-scale three-dimensional model construction has integrated a plethora of geological, geochemical, and geophysical data and led to a much better understanding of the tectonic and volcanic architecture of this ancient greenstone belt. The disseminated MKD5 orebody occupies a curvilinear thickening in the olivine adcumulate core of the host Mount Keith ultramafic unit. The orebody plunges shallowly to the south from surface, over a distance of 3.5 km, before bending sharply to plunge steeply to the north-northwest at depth. The thickest part of the orebody and the best Ni grades are associated with the bend in the overall ore trend. This also marks the approximate intersection point of two growth faults that appear to control the thickness variations in the adcumulate core and the plunge variations in the ore trend. The olivine adcumulate core is mantled by olivine orthocumulate rocks, which are best developed above and below the flanks of the orebody, and a fractionated sequence of pyroxenitic and gabbroic rocks occurs at the top of the Mount Keith ultramafic unit above the inside of the bend in the ore trend. These observations suggest a strong volcanological control on the position of the MKD5 orebody, and the local curvilinear thickening in the adcumulate core is interpreted as a fluid pathway, the locus of which was in turn controlled by the intersection of two broadly orthogonal growth faults. The Cliffs ultramafic unit consists of a lower olivine ortho- to mesocumulate unit, which varies in thickness along strike, overlain by a sequence of thin, spinifex-textured komatiite flow units intercalated with variably sulfidic tuffaceous shales. The Cliffs massive Ni sulfide deposit occurs at the base of a relative thickening (to ~100 m) in the lower olivine ortho- to mesocumulate unit. The orebody is typically less than 6 m thick, 500 m wide and plunges gently south from surface over a distance of 1.5 km, terminating in a structurally complex zone across which there are marked differences in the internal stratigraphy and thickness of the Cliffs ultramafic unit. The orebody has relatively low tenor (maximum ~8% Ni, typically ~5% Ni) and the tenor is strongly zoned with extremely low tenors (1–3% Ni) developed on the flanks. A barren exhalative massive Fe sulfide unit is typically present at the base of the Cliffs ultramafic unit on the flanks of the orebody and probably acted as a source of sulfur. The tenor zonation pattern suggests that the melting and entrainment of the underlying exhalative sulfide horizon occurred more vigorously in the center of the lava pathway. The structurally complex zone at the southern end of the Cliffs orebody is interpreted as a possible vent location; this would explain the relatively low tenor of the orebody and the tenor zonation (through limited mixing and entrainment of melted exhalative iron sulfide melt with the komatiite lava), the poorly incised nature of the lava pathway and the abrupt change in internal komatiite stratigraphy across this zone. There is good evidence that the southern terminations of both the MKD5 and the Cliffs orebodies are marked by growth faults. At a belt scale these points correspond roughly with the intersection of NNE- and NW-trending lineaments. These lineaments are on the order of 5 to 10 km in strike length and are visible in regional geophysical datasets (both gravity and magnetics). They are likely to mark the position of accretionary structures that were active at the time of volcanism and that probably controlled the volcanic architecture of the komatiite flow field and the location of Ni sulfide accumulations within komatiite lava pathways. Repeated reactivation of these early crustal-scale structures during subsequent orogenesis resulted in their upward propagation as sets of late brittle structures. Of broader significance to the understanding of the generation of komatiite-associated nickel sulfide deposits is the conclusion that many such deposits may be developed proximal to vent rather than in a distal setting, as has been proposed for the Kambalda camp.

Iron sulfide stoichiometry as a monitor of sulfur fugacity in gas-mixing experiments

We explore a method to utilize the stoichiometry of iron sulfide to determine the sulfur fugacity in experiments containing CO-CO 2 -SO 2 gas mixtures. The Fe-S phase diagram shows that the stoichiometry of iron sulfide melts is closely related to the sulfur fugacity ƒs 2 at a given temperature and ambient pressure. We derive equations that relate the sulfur fugacity to the mole fraction of sulfur X s in the iron sulfide from available literature data, and a solution model using the Redlich-Kister approximation for the excess Gibbs energy of mixing. We test the method by exposing iron sulfide to CO-CO 2 -SO 2 gas mixtures and subsequently analyzing the “Fe-S monitor” for its stoichiometry. Most sulfur fugacities calculated from the equilibrium gas composition agree within 5-10% (1200 °C) and 1-4% (1400 °C) with those derived from the sulfur mole fraction in the monitor and literature data calibration, which is consistent with the spread observed in literature data. There were no suitable literature data for a full calibration at 1300 °C, so we combined the available literature and our data to find the sulfur fugacity as a function of mole fraction sulfur. Overall the Fe-S monitor technique is a convenient method to determine the sulfur fugacity in high-temperature experiments containing CO-CO 2 -SO 2 gas mixtures as long as oxygen fugacities remain below that of the iron-wüstite or iron-magnetite buffer.

Formation of gold–silver sulfides and native gold in Fe–Ag–Au–S system

We carried out experiments on crystallization of Fe-containing melts FeS2Ag0.1–0.1xAu0.1x (x = 0.05, 0.2, 0.4, and 0.8) with Ag/Au weight ratios from 10 to 0.1. Mixtures prepared from elements in corresponding proportions were heated in evacuated quartz ampoules to 1050 ºC and kept at this temperature for 12 h; then they were cooled to 150 ºC, annealed for 30 days, and cooled to room temperature. The solid-phase products were studied by optical and electron microscopy and X-ray spectroscopy. The crystallization products were mainly from iron sulfides: monoclinic pyrrhotite (Fe0.47S0.53 or Fe7S8) and pyrite (Fe0.99S2.01). Gold–silver sulfides (low-temperature modifications) are present in all synthesized samples. Depending on Ag/Au, the following sulfides are produced: acanthite (Ag/Au = 10), solid solutions Ag2–xAuxS (Ag/Au = 10, 2), uytenbogaardtite (Ag/Au = 2, 0.75), and petrovskaite (Ag/Au = 0.75, 0.12). They contain iron impurities (up to 3.3 wt.%). Xenomorphic micro- (<1–5 μm) and macrograins (5–50 μm) of Au–Ag sulfides are localized in pyrite or between the grains of pyrite and pyrrhotite. High-fineness gold was detected in the samples with initial ratio Ag/Au ≤ 2. It is present as fine and large rounded microinclusions or as intergrowths with Au–Ag sulfides in pyrite or, more seldom, at the boundary of pyrite and pyrrhotite grains. This gold contains up to 5.7 wt.% Fe. Based on the sample textures and phase relations, a sequence of their crystallization was determined. At ~1050 ºC, there are probably iron sulfide melt L1 (Fe,S ≫ Ag,Au), gold–silver sulfide melt L2 (Au,Ag,S ≫ Fe), and liquid sulfur LS. On cooling, melt L1 produces pyrrhotite; further cooling leads to the crystallization of high-fineness gold (macrograins from L1 and micrograins from L2) and Au–Ag sulfides (micrograins from L1 and macrograins from L2). Pyrite crystallizes after gold–silver sulfides by the peritectic reaction FeS + LS = FeS2 at ~743 ºC. Elemental sulfur is the last to crystallize. Gold–silver sulfides are stable and dominate over native gold and silver, especially in pyrite-containing ores with high Ag/Au ratios.

Percolation threshold of iron-sulfide melts in olivine matrix: An experimental study with a centrifuging piston cylinder

There are several theories about metal-silicate segregation and the formation of the Earth’s core. One of them is the percolation of iron-sulfide melts through a solid silicate matrix, but the efficiency of such a mechanism is still under discussion. The main goal of the present investigation is to determine the percolation threshold for a simplified composition of the Earth core and silicate mantle and to verify whether buoyancy driven segregation of such metal-rich (anion-poor) melts is feasible. Starting materials are mixtures of natural olivine (S. Carlos) and iron-sulfide powder with the eutectic composition of the Fe–FeS system. The experimental apparati used are a standard end-loaded piston cylinder and a newly developed centrifuging piston cylinder. This rotating piston cylinder spins to a maximum speed of 2900 rpm (equivalent to an acceleration of 3000 g) at experimental conditions to 1.5 GPa and 1300 °C (standard 14 mm diameter, salt–pyrex assembly).

Metal‐silicate segregation in deforming dunitic rocks

We report the results of an investigation of the microstructural evolution of rocks composed of a silicate plus a molten metal sulfide phase deformed plastically in simple shear. Deformation experiments on samples of San Carlos olivine (Fo90) + 3, 5, or 9 vol% iron sulfide reveal the segregation of iron sulfide melt from the solid polycrystalline olivine matrix into regions enriched in metallic melt separated by regions depleted in the metallic melt. Previously published experimental studies on core‐composition metals in silicate rocks, performed under hydrostatic pressure conditions, suggested that metallic melts could not segregate from silicates by porous flow below a critical melt fraction of ∼0.05. In contrast, we demonstrate that metallic melts can segregate from solid silicates by grain boundary percolation assisted by deformation, down to a pinch‐off melt fraction of ∼0.01. The segregated metallic melt can accumulate into bands containing melt fractions exceeding 0.3. Using a model network of melt‐rich sheets with melt fractions of 0.3, calculated Earth‐scale permeabilities are as high as 10−9 m2. Segregation velocities for these sulfide melts can be up to 150 km/yr, fast enough for formation of Earth's core to proceed by deformation‐induced segregation well within the geochemically determined timescale of 30 million years. Therefore plastic deformation can facilitate removal of large enough quantities of metallic liquid from a solid silicate mantle to satisfy the geochemical constraints of the inefficient core formation theory without invoking the presence of a magma ocean.

Textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios

The influence of the coexistence of silicate and iron sulfide melts on the wetting of mantle olivine by iron sulfide melt is investigated. The dihedral angle between iron alloy and olivine is unaffected by the presence of silicate melt at 1.5 GPa and 1370–1410°C and is much greater than the maximum allowable dihedral angle for melt connectivity at low melt fractions. In experimental charges where silicate and sulfide melt occupy the same melt pocket, the sulfide melt adheres to olivine grain faces rather than forming discrete droplets. The relative interfacial energy between iron sulfide and basaltic silicate melt is lower than that of iron sulfide with olivine, but the contrast is not sufficient for basaltic silicate melt to wet FeS‐olivine grain boundaries. This result implies that very large silicate melt fractions are necessary to mobilize iron sulfide melts in partially molten silicate phase assemblages. Therefore segregation of Fe‐rich metallic melts by porous flow at low melt fraction is not a likely process for metal separation. The experiments also investigated the influence of the presence of a thermal gradient on sulfide melt migration. Although the sulfide melt can not migrate through the olivine network by porous flow, evidence in the experiments indicates mobility of sulfide through the interconnected silicate melt network. The sulfide melt pools in contact with silicate melt at the hot, top part of the experimental charges grow over time at the expense of the silicate melt pools in the cooler part of the charge. This points to a diffusive transport mechanism for local sulfide redistribution. We review literature data on metal‐olivine wetting angles and find that the metal composition has a dominant influence on the wetting behavior of a metallic liquid on an olivine matrix. Iron‐rich metallic liquid compositions are characterized by large dihedral angles (&gt;90°). The metal‐olivine dihedral angle decreases with increasing amounts of dissolved light element (S, C, O). For light element contents in excess of ∼59 at %, the dihedral angle approaches 60°, and the wetting behavior of a metallic liquid is similar to that of a silicate liquid.

Chemical bonding and the incorporation of potassium into the Earth's core

It has been argued that most of the Earth's potassium was segregated into the outer core and that the radioactive decay of 40K provided the heat source for the geodynamo. This idea rests on the assumption that the affinity of potassium for sulfur or metallic iron is enhanced at high pressure. In this paper, the high pressure electronic structures of K in sulfide, iron sulfide and metallic iron coordination environments were determined from molecular orbital (SCF‐Xα‐SW) calculations on (KS8)15−, (KS8Fe6)3− and KFe14 clusters. It is shown that, even at high pressure, potassium cannot alloy with metallic iron. Although a high‐pressure electronic transition may enhance the potassium‐sulfur chemical bond, the electronic structure of the KS8Fe6 cluster shows that this electronic transition cannot happen in an iron sulfide melt. Consequently, potassium will not have an enhanced affinity for sulfur in planetary interiors. If the lower mantle were more reducing, potassium might be excluded from the silicate phases by more strongly lithophile elements and segregated into a metal sulfide phase in the outer core (cf. the phase assemblages in enstatite chondrites). Given the oxidation state of the Earth, however, it is unlikely that significant quantities of potassium have been incorporated into the outer core. The Earth, like the moon and the eucrite parent body, is depleted in potassium. An alternative heat source (e.g., the radioactive decay of U and Th) must be invoked to explain the geodynamo.

Shock compression of KFeS2 and the question of potassium in the core

The Hugoniot equation of state of KFeS2 (initial density 2.663 g/cm3) has been determined for pressures up to 110 GPa. The Hugoniot data demonstrate a transformation at 13±1 GPa to a phase with an apparent zero‐pressure density of 3.7±0.2 g/cm3. A comparison of the inferred isentrope of KFeS2 (high‐pressure phase) with those of Fe, FeS0.9, and FeS2 indicates that the atomic volume of potassium in KFeS2 is approximately twice that of iron at 75 GPa. In the temperature and pressure range of the experiments, potassium fails to meet the empirical Hume‐Rothery and Raynor (HRR) criterion for solubility of an element in iron, namely, that the molar volume of the element should not exceed that of iron by a factor greater than ∼1.4. However, both the applicability of the HRR solubility criterion and the inferred isentrope of KFeS2 at high pressure are uncertain. Thermochemical calculations of the partitioning of K between a sulfide and silicate phase (e.g., KFeS2 and KAlSiO4 or KAlSi3O8 (hollandite)) indicate that pressure does not have a pronounced effect on the relative stability of solid KFeS2 and potassium aluminosilicate high‐pressure phases. The calculations suggest that the high‐pressure phase of KFeS2 would not be stable in relation to KAlSiO4 (kalsilite) in the upper mantle, or in relation to KAlSi3O8 (hollandite) in the lower mantle. However, the calculations do not bear directly on the question of partitioning of K into an iron sulfide melt from lower mantle aluminosilicate phases. Although the present results cannot absolutely rule out the hypothesis that a large fraction of the terrestrial potassium budget has dissolved into a molten iron sulfide‐bearing core, the present analysis of the pressure‐volume relation for potassium, iron, iron sulfides, potassium aluminosilicate, and potassium iron sulfide yields no evidence in support of this hypothesis.

The effect of pressure on the physics and chemistry of potassium

The equation of state and electronic structure of body‐centered cubic potassium are investigated theoretically by the quantum‐mechanical SAPW method. The results predict a series of pressure‐induced electronic phase changes brought about by the sequential filling of initially unoccupied d‐like electronic states. In addition to the discontinuous density changes that accompany these phase transitions, the presence of the d‐like bands makes potassium extremely compressible. At pressures as low as 500 kb (0°K), the ionic radius of potassium becomes compatible with that of iron and its electronic structure becomes like that of a typical transition metal. These properties should greatly enhance the miscibility of potassium in iron or iron‐sulfide melts.